Nearly every commercial electrically addressable liquid crystal display (LCD) manufactured and sold today utilizes glass substrates and the remaining few utilize small rigid plastic substrates. In all cases, the liquid crystal material is sandwiched between two such spaced substrates. The spacing between the substrates is generally controlled by either glass or plastic spacers that are often of a spherical or cylindrical shape or posts that are fixed to one substrate. Lack of flexibility and durability provide significant challenges in the manufacture of LCDs. Both challenges can be alleviated by using flexible plastic substrates. However, pressure points remain a significant problem.
Force applied to one of the substrates, commonly referred to as a pressure point, causes a deformation and nonuniformity in the cell spacing. As the gap between the substrates becomes smaller due to applied localized pressure, the liquid crystal flows radially out of the area. A resulting texture is apparent on the display due to the flowing liquid crystal. In a typical flat-panel computer LCD, this is an easily observable effect as the liquid crystal flows upon touching the screen and is visible by concentric rings of brighter and darker areas flowing away from the pressure point like ripples created on the surface of water as it is disturbed. Pressure points created in typical LCDs are an annoyance, but in operation the distortions in the liquid crystal director are transient and quickly relax back out of the structure due to the applied electric field used to refresh the pixel. Typical LCDs such as twisted nematic, supertwisted nematic, and in-plane switched must be constantly updated with an electric field to be switched as these liquid crystal materials are not bistable. As used herein, the term “stable” refers to the ability of a cholesteric or chiral nematic liquid crystal texture to remain even after removal of an applied electric field or other physical phenomenon such as pressure, magnetic field or the like. Bistable means that there are two liquid crystal textures or states that remain after removal of an applied electric field. In most instances, the bistable textures are focal conic and planar, wherein the focal conic texture scatters impinging light and the planar texture reflects the impinging light.
Cholesteric liquid crystal displays (ChLCDs) are bistable reflective LCDs that exhibit high reflectance and low power consumption. As such, they are suitable for numerous applications from small handheld devices to large area signage. Indeed, the most recent area of application for these displays is in high resolution electronic books where the technology can be applied with a low-cost passive matrix. As with most commercial LCDs, ChLCDs are typically made on glass substrates and often use a thick protective glass substrate to prevent pressure points. The need and desire to use plastic substrates in place of the typical glass substrates has been present since the beginning of volume LCD manufacturing. However, this change requires significant effort. ChLCDs lend themselves to conform to plastic substrates very easily. They do not require polarizers, there is no condition for non-birefringent substrates, and they do not necessarily require precise anchoring alignment control as do twisted nematic or super-twisted nematic LCDs. These and other features are strong motivating factors in the development of light weight flexible cholesteric displays.
The cholesteric material can be electrically switched to either one of two textures, planar or focal conic. In the planar texture, the director of the LC lies parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high thus the helical domain size becomes small and randomized in orientation such that it is just forward scattering and does not reflect impingent light. Once the defect structures are created in the liquid crystal phase, they are topologically stable and cannot be removed unless by some external force such as an electric field. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. These bistable structures can be electronically switched between each other at rapid rates (on the order of milliseconds). Gray scale is also available within a single pixel by applying intermediate voltages between the planar and focal conic states to adjust the ratio of reflective helical domains that are oriented perpendicular to the substrates (planar texture) to the randomized forward scattering domains (focal conic texture).
Bistable cholesteric liquid crystal displays have several important electronic drive features that other bistable reflective technologies do not. Of extreme importance for addressing a matrix of many pixels in a display is the characteristic of a voltage threshold. A threshold is essential for multiplexing a row/column matrix without the need of an expensive active matrix which requires a transistor at each pixel. Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology. And gray scale capability allows stacked red-green-blue (RGB), high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated.
Another important feature of cholesteric materials is that the RGB colors, as well as infrared (IR) night-vision, can be stacked (layered) on top of each other without optically interfering with each other. This makes maximum use of the display surface for reflection and hence brightness. This feature is not provided by traditional displays where the display is broken into pixels of different colors and only one-third of the incident light is reflected. Using all available light is important in observing a reflective display in a dimly lit room without a backlight. Because a cholesteric display cell does not require polarizers, low cost birefringence plastic substrates such as polyethylene terephthlate (PET) can be used. Other features, such as wide viewing-angles and wide operating temperature ranges as well as fast response times make the cholesteric technology the bistable reflective technology of choice for many applications.
Pressure points pose a significant challenge in creating a practical bistable cholesteric liquid crystal display. In bistable cholesteric liquid crystal displays, pressure points commonly appear as bright areas in the dark state and are highly visible until the pixel is refreshed. The only way to address this problem with known technology is to prevent the formation of pressure points in the first place by using a thick rigid protective glass substrate over top of the display or to encapsulate the liquid crystal with a polymer. Polymer encapsulation may be achieved through either emulsification or phase separation, as with polymer dispersed liquid crystal (PDLC). PDLC displays can be made from the following phase separation processes; polymerization induced phase separation (PIPS), thermally-induced phase separation (TIPS), and solvent-induced phase separation (SIPS). In the PIPS process, the polymer separates from the liquid crystal during polymerization as the molecular chain length increases. For TIPS, the liquid crystal is mixed with a thermoplastic polymer in a melt. As the melt cools, the polymer begins to solidify causing the liquid crystal to phase separate. An example of the TIPS process is disclosed in U.S. Pat. No. 6,061,107. The SIPS process involves mixing the liquid crystal, polymer, and an organic solvent to the single phase. When the film is cast, the solvent is allowed to evaporate out causing the liquid crystal to phase separate from the polymer. And although manufacture of polymer dispersed liquid crystal displays using plastic substrates with the TIPS process has been suggested, their manner of construction using thermoplastic polymer materials limits their use in flexible displays. Such a construction provides liquid crystal materials mixed with polymer materials that have a relatively low glass transition temperature. Accordingly, with only a minimum amount of heat applied, the polymer and liquid crystal material would likely revert to a homogeneous single phase causing the display to at least be rendered momentarily inoperative. And un-controlled re-cooling of such a display would likely result in droplets that are either too small (rapid cooling) or too large (slow cooling) for the prescribed electro-optic switching parameters as well as reducing or destroying the desired optical behavior. In compositions where the encapsulated TIPS display had adjacent liquid crystal droplets pitch changed through UV radiation (to make multi-colored pixels), reheating the composition to the single phase would destroy the multi-color display as all the different pitch chiral materials would become intermixed into a single pitch. Therefore, there is a need in the art for cholesteric liquid crystal displays with all of their aforementioned benefits but which can be practically implemented with flexible plastic substrates.